Hi !
IMO the best option is to put your hands on this: A monograph of the colubrid snakes of the genus Elaphe, FitzingerIt would be published in 1996 and is taxonomically outdated, hence the title, because its author, Klaus-Dieter Schulz, had yet to complete the revisions now in use and to describe formally a few then-unnamed subspecies (e.g. the Vietnamese blue beauty). As it is, it remains second to none though. If you are understandably not ready to pay a high price for one of the scarce copies currently available for sale, you should find it in a nearby library with a decent natural history department. I bet you won’t be disappointed.
Hope that helps.